PROBLEM QUESTIONS 89 



water. Place the jar in the sunUght for a day or two and collect 

 any escaping gas in the test tube. (See figure on page 95, Civic 

 Biology.) If carbon dioxide is occasionally passed from a gener- 

 ator through the water in the jar, the evolution of the gas is 

 increased. Place a glowing splinter in the gas. What happens? 

 What does this indicate? Set up the apparatus again and let it 

 stand overnight. Has any gas been formed ? 



Note. — The process of starch formation in green leaves in sunlight is called 

 photosynthesis (photo — light, and synthesis — a building up) . Chemically, water 

 (H2O) and carbon dioxide (CO2) are built up by the energy of the sun into a sub- 

 stance which ultimately becomes starch (CiHwOi). During this process oxygen 

 gas is given off as a by-product. 



Conclusion. — 1. If work is done by the plant, then how does it 

 use oxygen ? 



2. What gas would a green plant give off at night? Explain. 



3. What gas would be given off in the sunlight? 



Problem Questions 



1. What substances are given off by green leaves? 



2. Trace the pathway of water in a dicotyledonous plant from 

 the time it enters to the time it leaves the plant. 



3. What does a plant do with the water it takes in ? The gases 

 of the air that enter the leaves ? The mineral matter that passes 

 in through the roots ? 



4. If dead plants are burned in the fall, does as much raw food 

 material get back in the soU as if the dead bodies were plowed 

 under? 



5. Why are green plants said to be constructive while animals 

 are said to be destructive? 



6. Compare a leaf to a factory. Where does the energy to run 

 the plant come from? 



7. Why should the leaves of plants in our homes be frequently 

 dusted and washed ? 



8. Why do the leaves of lettuce or cabbage when " headed " 

 turn white ? 



9. What are some adaptations in leaves to receive light? 

 10. Find some ways in which leaves are protected. 



